Frequency modulation



AUDIO INPUT July 29, 1947, Y -E. w. KENEFAKE 2,

FREQUENCY MODULATION I Filed May :50, 1944- AUDIO INPUT Inventor" v Edwin W. Kenefake,

y His Attorney.

Patented July 2 9, 1947 FREQUENCY MODULATION Edwin W. Kenefake, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application May 30, 1944, Serial No. 538,026 7 2 Claims. 1

The present invention relates to frequency modulation transmitters and is particularly concerned with preventing overmodulation.

The object of my invention is to provide a control for preventing overmodulation which permits higher modulation percentages in the normal operating range.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is a diagram of a frequency modulation oscillator having a modulation control embodying my invention; Fig, 2 is a diagram explaining the operation of the phase shift network supplying the reactance tubes; and Fig. 3 is a diagram explaining the performance of the modulation control.

Referring to the drawing there is shown an oscillator I having a frequency determining circuit comprising an inductance 2 and a shunt condenser 3. Operating potential for the oscillator is supplied in conventional manner from the upper terminal 43 of a power supply potentiometer 44 which is connected to a suitable source of 3-]- operating potential (not shown). Associated with the frequency determining circuit are push-pull reactancetubes 4, .5 having anodesli, 1 excited in phase with the oscillator output from one end of the inductance 2 and having grids 8, 9 excited in phase quadrature with the oscillator output from diametral terminals Ill, ll of a phase shift network comprising a, bridge I2 having equal impedance arms consisting of resistances l3, I l and condensers l5, l6. Intermediate terminals |8 of the bridge are connected to taps on the inductance 2 on opposite sides of a center tap 2a connected to the power supply. Equal currents flow through the arms of the bridge between terminals l7, |8 which, as shown in Fig. 2, lead the voltage between the terminals by 45. The volta e E11 at terminal II is therefore equal and opposite to the Voltage E at terminal Ill and both voltages are displaced in phase exactly 90 from the output of the oscillator appearing across the inductance 2. The tubes 4 and 5 accordingly draw reactive current which simulates a reactance connected across the frequency determining circuit of the oscillator.

The amount of reactive current through the tubes 4, 5 (and hence the variation in the oscillatorfrequency) is controlled by varying the potential on the grids 8, 9 in accordance with an audio signal coupled to'the grids 8, 9through blocking condensers 45 and 46 from output terminals |9, 20 of a push-pullaudio amplifier. The amplifier comprises devices 2|, 22 having anodes 23, 2d connected to the terminals [9, 20 and grids 25, 26 connected to the secondary 21 of .a transformer having a primary 28 connected to the source of audio signals (not shown, .but indicated by Audio input). With this arrangement, the potential on the grids 8, -9 varies in magnitude and frequency with the amplitude and frequency of the audio signals to modulate the frequency of the oscillator by causing the oscillator frequency to vary or deviate from'the mean frequency in accordance with the audio signals,

In frequency modulation, the maximum deviation of the transmitted frequency must be restricted to avoiddistortion at the receiver. The frequency deviation should, however, be as large as possible since the signal intensity increases with the magnitude of the frequency deviation.

Frequency deviation is equivalent to modulation percentage in amplitude modulation.

The frequency deviation is restricted (overmodulation is prevented) by an over-modulation voltage developed by an automatic volume control comprising diodes 29, 30 connected between the audio amplifier output terminals I9, 20 and a source 3| of voltage positive with respect to ground which'dete-rmines the point at which the automatic volume control becomes effective. Positive half cycles of the audio signals of greater voltage than the voltage 3| cause diodes 29, 30 to draw current through condensers 41, 48, thereby to develop negative voltages with respect to ground at junction 49 between resistances 32, 33. These voltages are applied through a filter 34, 34a to a time constant network consisting of a condenser 35 shunted by a resistance 36. Ihe voltage of the condenser 35 is coupled through conventional loading resistances 31 and 31a and primary winding 21 to the audio amplifier grids 25, 26 reducing the audio amplifier output. This reduces the audio signal voltages applied to the grids 8, 9 of the reactance tubes and tends to decrease the effective reactance of the tubes. A portion of the voltage of the condenser 35 determined by the adjustment of a slider 38 on the resistance 38 is applied through resistances 39, 40 to the grids 8, 9 of the reactance tubes, reducing the reactive current through the tubes and further decreasing the effective reactance. The over-modulation control voltages applied to the audio amplifier termining circuit of the oscillator.

and to the reactance tubes both tend to decrease the frequency deviation (the modulation percentage) for strong signals b decreasing the eifective reactance connected across the frequency de- Other arrangements for obtaining over-modulation control voltages are known.

The performance of the modulation control is illustrated in Fig. 3. Curve ll is typical of the modulation control obtainable by applying the over-modulation control voltage to either the audio amplifier or to the reactance tubes. The curve has a relatively steep slope t point A at which the automatic volume control becomes effective (when the positive half cycles of the audio signals exceed the voltage 30., .As the audio input increases, the automatic volume control voltage reduces the modulation, but at very large signal inputs the modulation exceeds the desired limit. In order that this type of control be effective, the point A must correspond to a relatively low modulation percentage so the transmitter is not as efiectively used in the normal operating range. Curve 42 is typical of the modulation control obtainable by applying the overmodulation volume control voltage to both the audi amplifier and the reactance tubes. This curve has a steeper slope to point B, the point at which the automatic volume control becomes effective, and thereafter remains substantially constant corresponding to a constant modulation percentage. The modulation percentage for very strong signals is kept below the desired limit. It will be noted that the modulation percentage for low signal inputs is higher than for curve 4| and the transmitter is therefore used more effectively to transmit information in the normal operating range.

It is obvious that the curves Al, 42 are subject to modification. .For equivalent designs, the.

curve 42 always exhibits better performance than the curve 4!.

While I have shown particular embodiments of my invention, it will be .understood that many modifications may be made without departing from the spirit thereof, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination, in a frequency modulation transmitter, of a source of carrier wave oscillations to be modulated, a reactance tube modulator having a grid and cathode, said modulator being connected to said source to vary the frequency thereof in accordance with the potential between said grid and cathode, an audio amplifier, means to supply audio signal oscillations through said amplifier to said grid and cathode of said modulator, means to rectify the audio signal output from said audio amplifier when it exceeds a predetermined level to produce'unidirectional control potentials, and two unidirectional current circuits one connected to impress said control potentials between said grid and cathode of said modulator with polarity to reduce the extent of modulation produced thereby and the other connected to impress said unidirectional potentials upon said audio amplifier with polarity to reduce the amplification thereof.

2. In a frequency modulationtransmitter, an audio amplifier having a grid circuit coupled to a source of modulation signals and an anode circuit, a reactance tube modulator having an input circuit coupled to said anode circuit and an output circuit, an oscillator having a frequency-determining element interconnected with said output circuit, said oscillator being frequency-modulated by modulation signals supplied from said source through said amplifier to said modulator, a modulation signal detector connected to said anode circuit, said detector having a delay bias thereon and developing negative unidirectional control potentials by rectification of the amplified modulation signals only when said signals exceed said bias, and circuits for simultaneously impressing said potentials back upon said grid circuit and forward upon said input circuit to reduce both the audio gain and the degree of frequency modulation, respectively.

EDWIN W. KENEFAKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

